Not Applicable
1. Technical Field
The present invention relates to the field of injection molding, and more specifically to a method of preparing a mold suitable for forming plastic optic components.
2. Background Art
Thick non-imaging optics, such as collimation and concentration non-imaging optics, are expensive because they are typically fabricated by machining glass. These thick non-imaging optics are typically characterized by a high aspect ratio and a number of extremely tight tolerances, including surface finish, flatness, concentricity, and perpendicularity. The cost of these optic elements can be reduced considerably by replacing the machined glass components with molded plastic components.
It is known in the art that such plastic optic elements can be molded, for example by injection molding, when the part geometry lends itself to molding. It is also known that manufacturing the molds for high aspect ratio plastic optic parts is extremely difficult.
There is a need for a method of producing molds suitable for injection molding fabrication of plastic optic components.
Our invention is a method for manufacturing molds suitable for the fabrication of optical components using injection-molded plastic. The injection molding materials that are suitable for use with our invention include but are not limited to cyclo-olefin polymer, clear acrylic resin, and polystyrene.
To fabricate the injection mold for a plastic optic element in accordance with our invention, metal models of the plastic optic element are first carefully machined to the exact dimensions of the desired plastic optic elements, but with adjustments to the dimensions to compensate for the determined rate of shrinkage, during curing, of the plastic when it is injected into the finished mold. Specifically, to fabricate the injection mold for the plastic optic part, two such machined models are employed. Each of these metal models then has coated thereon shells, as by electroforming, which shells conform exactly to the dimensions of the metal models. In further embodiments of our invention, more than two machined models and their corresponding shells are employed.
In accordance with an aspect of our invention, each shell including its encased metal model is then scribed on its outside with two lines, the first defining an axial line which, in one embodiment, splits the shell in two parts. Depending on the shape of the plastic optic element to be molded, the axial line may not split the shell into two equal size parts. The second scribed line is offset slightly from the first, being below the axial line for a first of the shells and above the axial line for the other of these shells. The shells are then cut along these second scribed lines, and the portion of the metal model away from the axial line and between the second cutting scribe line together with the outer shell formed thereon is then discarded.
What remains are two shell sections slightly larger than the dimensions of the desired plastic optic part with a part of the metal model still in these shell sections. Accordingly, in the next step, the remaining part of the metal model in the shell together with that portion of the shell between the axial line and the second cutting scribed line are then removed from each shell section. While the cutting operation along the scribed cutting lines may distort the dimensions of the outer shell adjacent to these cutting lines, in accordance with our invention, because the cutting of the shell is done away from the axial line, the cutting operation will not affect the desired dimensions of the finished mold when the two mold half shell sections are then joined together to form the finished injection mold.